1. Field of the Invention
The present invention relates to a photocathode apparatus for emitting photoelectrons.
2. Description of the Related Art
Generally, a photocathode apparatus has been used as a photoelectric surface of a photomultiplier tube, an electron beam source of a large-scale accelerator, a bright electron beam generating apparatus or an image pickup apparatus.
A first prior art photocathode apparatus includes a cathode made of metal such as Au or Cu, or semiconductor such as GaAs. In this first prior art photocathode apparatus, the irradiation surface of the cathode is irradiated with photons having an energy larger than the work function of the cathode, photoelectrons are emitted from the irradiation surface of the cathode due to the photoelectric effect.
In the above-described first prior art photocathode apparatus, however, since the reflectivity of the cathode is very high, the ratio of the number of photoelectrons emitted from the irradiation surface of the cathode to the number of photons incident thereto, i.e., the quantum efficiency η is very low or about 10−3 to 10−4. 
In a second prior art photocathode apparatus, the irradiation surface of the cathode of the first prior art photocathode apparatus is deposited by alkali metal such as Cs or alkali metal compound to decrease the work function of the cathode, which would increase the quantum efficiency η (see: JP-60-180052 A and JP-9-213204 A). In this case, the quantum efficiency η is high or about 10−1. That is, the quantum efficiency η of the second prior art photocathode apparatus is about 102 to 103 times that of the first prior art photocathode apparatus.
In the above-described second prior art photocathode apparatus, however, since the alkali metal or alkali metal compound on the irradiation surface is irradiated with intense light, the alkali metal or alkali metal compound would deteriorate, so that the lifetime of the apparatus would be shortened, for example, the lifetime would be about 100 hours.
Also, in the above-described second prior art photocathode apparatus, since the alkali metal or alkali metal compound is easily oxidized, the above-described second prior art photocathode apparatus must be operated in an ultra high vacuum state of 10−8 Pa, which would require ultra high vacuum equipment, thus increasing the manufacturing cost.